Delayed hypersensitivity to purified protein antigens in incomplete Freund's adjuvant can be suppressed in sensitized recipient guinea pigs by donor lymphoid cells from animals which are in a suppressed state, i.e., during the interval between definitive intradermal delayed hypersensitivity and the onset of Arthus reactions. Such intradermal delayed hypersensitivity can be suppressed in hypersensitive recipients with donor spleen cells in a suppressed state. The duration of delayed hypersensitivity varies with the dose of cyclophosphamide administered 72 hours before sensitization.